Transparent Cube For Character
You can accomplish this effect using material nodes, a mesh, a spot lamp, and a camera.
Blend-File (made in UPBGE 0.2x)
https://blend-exchange.com/b/bOl2J557
Explanation
When the spot lamp's shadow, which is casted by the mesh, hits the surface of the building, the casted shadow will act as an alpha (transparency) mask for the building.
Screenshot (nodes setup)

Tutorial
https://youtu.be/4Seqm826oWk